1. Field of the Invention
The present invention relates to a combined cycle power generating plant having a fluidized bed furnace having a heat exchanger disposed therein; a steam turbine which is driven by steam generated in the heat exchanger and a gas turbine which is driven by combustion gas generated in the fluidized bed furnace and a method for operating the same.
2. Related Art
A prior art power generating plant having a fluidized bed furnace having a heat exchanger disposed therein, a steam turbine which is driven by steam generated in the heat exchanger and a gas turbine which is driven by combustion gas generated in the fluidized bed furnace is disclosed in Japanese Unexamined Patent publication No. Tokkai-Hei 1-217108. This power generating plant includes a storage tank for storing a bed material therein as well as the fluidized bed furnace.
The fluidized bed furnace has recently attracted interest since it is capable of removing nitrogen oxides (NOx) and sulfur oxide (SOx). It is necessary to keep the temperature of the fluidized bed at the most appropriate temperature in such a manner that the removal reaction of the sulfur oxides and nitrogen oxides proceeds properly.
Therefore, in order to control the temperature in the fluidized bed so that it falls within a predetermined range of temperature, the contact area between the fluidized bed furnace and a heat exchanger (heat exchanging surface area) is changed by transferring a bed material between the fluidized bed furnace and the storage tank to change the bed height in the fluidized bed furnace. The heat transfer rate is thus increased or decreased for controlling the temperature of the fluidized bed.
If the output of the whole of the plant is considered, the plant is operated as follows:
The heat exchanging surface area of a heat exchanger is increased by elevating the bed height so that the amount of steam generated in the heat exchanger, that is, the amount of steam supplied to the steam turbine is increased for increasing the output of the steam turbine as shown in FIG. 9.
On the other hand, in order to increase the output of the gas turbine, it is necessary to elevate the temperature of the combustion gas and to increase the pressure of the combustion gas. However, the output of the gas turbine can not be actively increased since it is necessary to maintain the temperature of the combustion gas in a predetermined range Of temperature as mentioned above. Accordingly, increasing of the output of the gas turbine is achieved by increasing the pressure of the combustion gas. The pressure of the combustion gas,-that is, the pressure in the furnace is increased by increasing the amount of supplied fuel to elevate the bed height. However, the increase in the rate of the pressure in the furnace is slow as shown in FIG. 9. Accordingly, considering a single gas turbine, although the output thereof increases to some extent, the output of the gas turbine is directed to a compressor which is directly connected to the gas turbine. The output from the gas turbine to a component external of the system is not readily increased as shown in FIG. 10.
If the increase in output of the whole of the plant is achieved, the gas turbine output does not readily increase although the steam turbine output increases. Furthermore, since the change in the bed height of the fluidized bed furnace involves the movement of the bed material which is a powdery material, the response of the gas turbine is so low that the increase in the output of the steam turbine tends to be delayed. Therefore, there is the problem in that the response of the increase in the whole of the plant is slow.